CrossRoads:
I just did the same recently, with just 2 shift registers and 1 ULN2803, to control up to 128 mulitplexed LEDs.
I kept the data of what was being displayed on the LEDs in an array, and shift it out to the shift registers every 500uS using SPI.transfer()'s.
Whenever serial data comesin, the array is updated between refresh cycles.
sounds very interesting. 128 leds, cool : )
i had a go with an ardunio tutorial a few weeks ago (below) which works more efficiently than mine above. all the user needs to send is an 8-bit number from max and the arduino does the rest of the work. The arduino analogue inputs are not enabled in the sketch provided however, which is why I decided to make my own patch.
little-scale.blogspot.co.uk/2008/01/talking-to-3-595s-via-maxmsp.html